In human organism development is a very complex and highly regulated system that enables the functional balance of each organ in a whole body. Disorders and tumor micro-environment weaken host immune system that is not able to recognize the tumor as a unknown body and fight against its uncontrollable forces. Tumor avoids the immune system in a way that promotes immunosuppression and orientation cytokine production towards Th2 immune responses which are responsible for infection appearances. Some of infectious agents (viruses) can cause oncogene activation and inhibition of tumor suppressor genes. It is also known that oncology treatment can be detrimental to the host immune system. The drugs or radiation can activate different signaling pathways which lead to a vicious circle from which there is no return. Experimental models of tumor biology and molecular events in vivo are patients who have multiple primary cancers (MPC) diagnosed during life. Such patients confirm the complexity of disorders that occur in the cell and explain all the influences and contributions to developmental tumor cascade.